Differential Guide
The benefits of performance differentials

Every car has a differential in the drive train. The differential splits the power from the engine/gear box between the two driven wheels whilst allowing them to rotate at different (see where the name comes from?) speeds.

The information below concentrates on the characteristic of differentials when used in front wheel drive cars, like...ohh I dunno...VW Polo's?

Why is a differential required?

When turning a corner the driven wheels of a car must travel at different speeds. The reason for this is that the wheels on the outside of the car (the passenger side when turning right) must travel a greater distance than those on the inside (the drivers when turning right).

The outside wheel travels a greater distance but in the same period of time as the inside wheel, so it must turn faster (Who knew those maths/physics lessons on Speed = Distance/Time would proove useful for making your car better?)

A differential that behaves in the above manner is known as an 'open differential.'

Why should I consider a performance orientated differential?

The very nature of an open differential means that it has a number of draw backs when used in a performance car/situation. The power is always sent to the wheel with the least resistance. The wheel with least resistance is the wheel with the least traction. Applying too much power will cause this wheel to loose traction and slip. As the wheel slips the level of resistance will decrease and more power will be directed to that wheel.

Example: Imagine a car sitting on a road of two halves. The drivers side wheels are on dry tarmac. The passenger side wheels are on ice.

As the power is applied it is directed to the wheel with least resistance. In this case that will be the passenger side wheel in contact with the ice. As the driver accelerates the passenger side wheel looses traction and begins to slip. The level of resistance drops and more power is directed to that wheel. Speed drops and eventually all or most of the power is being wasted on the spinning wheel.

This is a rather extreme/unusual example but it illustrates how differing levels of grip/traction/resistance across the front axel can cause loss of grip and waste of engine power. The same principles are at work when accelerating on dry tarmac.

Common real world examples of where open differentials show weakness include: full bore standing starts (1/4 mile for example), accelerating out of a bend where the inside wheel has little or no weight over it reducing traction (power spun away rather than driving the car out of the corner), driving in slippery conditions (such as snow/ice or on greasy tarmac) and finally, loosing traction in a front wheel drive car when a wheel looses contact with the road (taking a lot of curb on a circuit when cornering).

If only there was a way to ensure the power was direct to the wheel on the dry tarmac...well there is! Enter the limited slip differential or LSD as it is commonly known.

What types of performance differentials are available?

DR Gripper

Multi-plate mechanical type differential

Plate (ie Gripper, Gemini)

• found in race paddocks and rally stages, wherever clocks are ticking and wherever the last 10th of a second counts
• results in heavy steering on non-PAS equipped cars
• snatchy, depending on preloads and ramp angles
• allows kerb hopping eg where those 10ths really matter:

Characteristics:

Acceleration
Deceleration

Supplier:

Polo Performance Parts

Quaife ATB

Automatic Torque Biasing Torsen type differential.

• Maintenance free (fit & forget)
• Proven design
• negligible effect on steering effort
• smooth progressive diff lock up
• if one wheel loses grip, the diff will unwind and not lock up. An ATB requires an element of grip from both wheels to operate. It can regain grip, and whilst
• losing grip the car remains totally driveable.
• inferior to a plate diff on any cornering which could give rise to the inside front wheel lifting from the ground (but if your inside wheel lifts, your suspension arguably is not set up to maxmise grip from the car)
• if a driveshaft snaps, the car behaves like an open diff, ie you cannot drive on one driveshaft

Characteristics:

Acceleration
Deceleration

Supplier:

Group Buy Thread

Which type of differential should I choose?

The type of differential you choose should reflect the type of driving you do and your expectations from your car. A differential that is suitable for motorsport may not be suitable for a road car and vice versa.

What other modifications should I consider?

Suspension

Wishbone Subframe

Uprated Drive Shaft

Uprated Clutch

Gearbox Rebuild

Can this get pinned in the FAQ when finished? It's a work in progress so feel free to add suggestions/corrections...

Plate diff > ATB on the the quarter mile, you tend to want a bit of wheelspin - so you need the diff to stay locked under those conditions. Also at RWYB especially, the surface isn't always spot on - so you get a bit of a 'split-mu' situation, one wheel's on a grippier surface than another, so the plate type diff again is preferable.

HOW MUCH UNDERSTEER DO YOU WANT ITS ONLY A POLO
you will get better traction from shocks and spring set up than spending £2500 on a gearbox and diff or spend £12000 on 7 speed sequencial fcuker

Yup def. multi-plate for 1/4 mile and serious circuit work I think. Going to stress that point in the first post...but also put the argument forward that for road driving and occasional visits to the track and strip the ATB may prove to be the better option due it's subtle nature. Taking info from the old plate vs. ATB thread.

Going to add some info on other suggested modifications...subframes, up-rated drive shafts and perhaps some other points?



Ok..you don't sound like an ass hole at all?

a) It's not just to do with understeer. Straight line traction and drive out of corners is a serious issue is Mk3 G40's running increase power and torque.

b) Springs and shocks will go some way to improving traction but they alone really aren't sufficient to reign in the engine.

c) Differentials are in the £400-500 region...so really aren't too expensive for the increased performance on offer.

no not an asshole
085 boxes are week in the bearing dept and gear dept so a good LSD will help you get off the line but if the clutch dont brake the box will explode or snap right hand shaft putting a big hole in your sump.Fit good suspension and springs and i mean good not cheap look to spend a grand and buy good tyres for track or strip
(sorry) this is not a rant
tuned g40 driver on road and trackdays

My G has hideous torque-steer. How much worse does it get with a LSD?

Bearings aren't really a problem as long as you're not expecting over 120,000 miles from them. 5th and 2nd gears along with 2nd gear synchros tend to be the main failures - along with the pinion gears in the standard diffs breaking up. I'm fairly sure all the high powered G40 boys have broken at least one 5th gear set each!

If you're fitting an LSD, it's sensible to run an uprated driveshaft to replace the thin off-side item as you will break it otherwise as you mentioned. The LSD tends to put more strain on the final drive bolts, but I think JP Race now have this covered with some more hardcore parts - I know a certain person with >250hp had a few issues with them using a Ricardo/Gemini diff and soft A048Rs.

If you're running an LSD, then I think you'd almost certainly have a beefier clutch in there anyway. Helix and Sachs paddles seem to take the abuse okay - I did some 6,500rpm 2nd gear launches to see if I could get my Helix to slip, and it's still in one piece...

Personally, I'd say uprated driveshaft, new final drive bolts, 'box rebuild (if it's not already had one) and an uprated clutch should be considered as a package.

I wouldn't say that a grand's worth of suspension is an alternative to an LSD though - if you're serious, you'll be doing both - and each of them can make the car quicker in slightly different ways.

Giorgio - with a plate type diff it can feel very snatchy. If you snap off the thottle mid-corner on the limit you'll get massive understeer, but keep on the power and the diff pulls the car round at speeds unachievable without one.

^^^ Will add the above into the post Andy.

Pics of the Grippers here Tav:

Grip Me

will send you pics of Hayseys broken Phase 1 if it's of any use too- he's just bought a Gripper so is kindly doing an installation guide so I can stick it on our site, if any pics are of use i'll pass them on. Also he's going from a Phase 1, to subframe and gripper diff, be an interesting comparison!

Cheers

Yoof

Excellent. More info the better. Just trying to make some useful FAQ's for people to use.